Selective call receivers are radio frequency (RF) receivers which selectively receive messages. Selective call messages are recovered from a selective call transmission by demodulating and decoding a selective call signal broadcast by a selective call network. Selective call signalling networks typically utilizes a binary frequency shift keying (FSK) modulation signalling protocol such as the POCSAG signalling code. POCSAG is a signalling protocol originally proposed by British Telecom and an anagram for the Post Office Code Standardization Advisory Group.
A POCSAG signal comprises a preamble having a number of zero-to-one transitions. The POCSAG protocol receivers perform bit synchronization on the transitions of the preamble. Bit synchronization is a process used to determine the presence of bit boundaries of a data transmission having bits transmitted at a predetermined baud rate, and thereafter to provide a clock to synchronously sample the bits. One method for POCSAG bit synchronization is described in U.S. Pat. No. 4,506,262.
The first word following the preamble in POCSAG signalling is a synchronization (sync) code word which is defined to contain a predetermined binary sequence used for frame synchronization. Frame synchronization frames the bits transmitted so that the data transmitted is in a form to be decoded, thereby indicating various word boundaries such as the first bit of address and data signals. The predetermined binary sequence is the defined POCSAG sync code word. Sixteen words of information, divided into eight frames, follow before the sync code is repeated. The sync code word provides a means for detection of frame synchronization. The combination of a sync code and the eight frames following is referred to as a batch.
If no frame synchronization has been detected within a period of time substantially greater than the time to transmit sixteen words (five minutes, for example), the POCSAG selective call receiver determines that it has lost the signal and begins searching for the preamble in a conventional manner.
If frame synchronization has been detected, the POCSAG selective call receiver looks in the following frames for its address and an associated message. Conventionally, a selective call receiver examines a predetermined one of the eight frames following the sync code to determine whether the receiver has been addressed. When an address assigned to the receiver is detected in the corresponding frame, the selective call receiver decodes a selective call message. The user is alerted that a message has been received and the message can thereafter be presented, either as a voice message or a numeric or alphanumeric displayed message.
More recently, users of selective call receivers desire additional features, such as reception of information services, while maintaining the desire for small portable receivers. Information services are services which provide regularly updated messages, such as news services (UPI), financial services (Dow Jones Stock Reports), or sports services. One such information services selective call system is described in U.S. Pat. No. 4,845,491.
A selective call receiver must necessarily process information service selective call messages different than personal selective call messages. Additionally, the information service messages may be longer than (and could possibly be substantially longer than) one frame of the POCSAG protocol. One method proposed for differentiating the two types of messages utilizes dual synchronization codes--one indicating personal messages follow, and the other indicating that information services messages follow. U.S. Pat. No. 4,783,654 describes an example of a dual sync code system. Some selective call systems have adopted a dual sync code protocol. Other selective call systems use the defined POCSAG sync code.
It is undesirable for a user to use two selective call receivers to be able to receive messages in dual sync code systems and conventional POCSAG sync code systems. Yet, conventional selective call receivers may only receive and decode signals of one type or the other.
Thus, what is needed is a method and apparatus for receiving selective call signals in a universal synchronization code operating mode wherein conventional POCSAG signals and signals utilizing dual synchronization codes can be received and decoded appropriately to recover personal and information services messages.